feat: add income range mode

[Goodness-0x]

Summary

Adds an inclusive income band mode to the income proof while keeping the existing threshold mode intact.

What changed

• Added range-mode support for income proofs: min <= income <= max
• Kept legacy threshold mode working: income >= threshold
• Wired min/max values through the witness builder and verification URL flow
• Validated income band inputs in the frontend and SDK
• Documented the public input layout for the income proof circuit

Why

This enables tiered eligibility checks such as subsidy or bracket-based income gates without revealing the exact value.

Testing

• Added focused validation for income range params
• Verified SDK URL generation for banded income claims
• Verified the validation suite passes

Closes #414

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[Psalmuel01]

The income range mode is implemented well (threshold mode preserved, plus min/max band with income >= min and income <= max, and mode-selection tests). Two things before merge. First, changing income_proof/src/main.nr changes the circuit and therefore its verification key, but there is no fixtures/income/vk update in this PR. Once merged that would fail main's Circuit compile artifact-freshness check (the committed VK would no longer match the circuit). Please regenerate and commit the income vk/proof/public_inputs from the updated circuit. Second, CI has not run on this branch yet (no checks reported) - push so it runs and confirm everything is green, especially the circuit job. Also confirm the witness/SDK path passes min and max through end to end.